The present invention relates to strain gages. A strain gage is a strain sensitive resistive device used to measure mechanical strain. A strain gage is typically adhesively bonded to a surface and then measured changes in the resistance of the strain gage are associated with various effects depending upon the configuration of the strain gage. Strain gages can be used to measure bending, axial and torsional load or other strain effects. A strain gage is made of a resistive foil which is typically photoetched, ion milled, or otherwise cut to form a pattern to produce a resistance. Foil material is usually a Cu—Ni or Ni—Cr alloy of 50 microinches to 200 microinches in thickness. A typical resistance value associated with a strain gage is 120 ohms. The foil pattern is usually bonded to a very thin flexible polymer backing with an epoxy or similar resin or other cement. The polymer backing is thin (0.5 to 1.0 mils) to enhance flexibility. Such a device is strain sensitive according to the formula:
                    k        r            ⁢                        d          ⁢                                          ⁢          l                l              =                  d        ⁢                                  ⁢        r            r        ,      where    ⁢                  ⁢                  d        ⁢                                  ⁢        l            l      is strain imposed on the gage when it is cemented to a structure under load (stress),
  dr  ris the relative resistance change due to the strain, and k is a constant. The constant k of the strain gage is the proportionality factor between the relative change of the resistance and strain in the gage. Sometimes k is called the gage factor. The constant k is typically approximately 2 for foil materials such as Cu—Ni or Ni—Cr.
Since the strain gage is very flexible, it can be applied to curved surfaces of very small radius. Because the gage is very flexible and “sticky” from static charge, it presents certain severe disadvantages.
One problem with a prior art strain gage relates to labor content in final manufacturing steps including:    1. Resistance checking,    2. Optical inspection, and    3. PackagingPrior art strain gages are typically packaged manually in tray pockets or plastic folders, resulting in cumbersome and costly handling requirements.
In other electrical component industries, use of automation is prevalent. As an example, modern electronic components are constructed to take advantage of automated systems utilizing vibrating bowl sorting equipment, which include automated resistance checking, optical inspection steps, and final tape and reel packaging.
The present invention attempts to improve upon the state-of-art by describing a strain gage capable of utilizing existing modern automated equipment commonly used in the electronic component industry, without significant performance reduction.
A further problem is that such a prior art strain gage is fragile during handling by hand or machine.
Yet another problem is that strain gages are difficult to install. Strain gages are cemented to the structure for which strain is measured. Electrical lead attachments must be made and generally such handling requirements create inconvenience and cost. Therefore, it is a primary object of the present invention to improve upon the state of the art.
It is another object of the present invention to provide a strain gage that can be automatically sorted and packaged on a tape.
Yet another object of the present invention is to provide a strain gage that need not be individually packaged.
A further object of the present invention is to provide a strain gage that is robust and easier to handle.
A still further object of the present invention is to provide a strain gage that is conducive to easy installation.
One or more of these and/or other objects, features, or advantages of the present invention will become apparent from the description and claims that follow.